Studies of adults indicate that circadian phase preference, or morningness/eveningness (M/E), although influenced by behavioral or life-style factors may be a constitutional or trait-like individual characteristic, perhaps under genetic control. This notion is supported by the association of M/E with phase-angle of entrainment, intrinsic period, and circadian amplitude. Developmental changes in adults include a shift to more morningness with increased age. During adolescent development, by contrast, M/E preference becomes more evening type. Models of the regulation of waking alertness include circadian and homeostatic sleep/wake factors, which we have shown to be prominent features regulating alertness in adolescents. We predict that entrained phase angle of the sleep/wake cycle affects the diurnal alertness pattern when nocturnal sleep is nominal or restricted. Study 1 tests the model by examining diurnal alertness (using Multiple Sleep Latency Test) in 64 adolescents (16 boys and 16 girls in each M/E group) whose M/E values are in the upper or lower 16% of a normal control sample. (Entrained phase angle will be assessed to determine whether adolescent morning and evening types sort on this variable in the same manner as adults.) We hypothesize that the pattern of diurnal alertness will show greater morning sleepiness and an evening rise in alertness for the evening types and that morning types will be alert across the day but become sleepier earlier in the evening with a steeper decline than seen in the evening types. Subsequent exposure to restricted nocturnal sleep (much as occurs in older adolescents) will be imposed for one week. Our hypothesis predicts that the evening types will be severely impaired in morning, yet show an afternoon-to-evening augmentation of alertness; morning types will manifest some morning impairment and will be considerably impaired in the late afternoon and especially the evening. Ad lib recovery sleep will be examined to determine how M/E affects extended sleep, as adolescents often experience on weekends. The role of pubertal development will be examined first by correlational analysis of puberty with M/E within M/E group and secondly with a longitudinal follow-up assessment (Study 2) in a subsample of adolescents in each group (4 each prepubertal and late pubertal from each group). Finally, Study 3 is a pilot study to assess whether intrinsic period is shorter in adolescents with morning than evening phase preference, as has been reported in adults.